Telephone substation circuits



1,624, 56 April F. w. MCKOWN ET'AL Q TELEPHONE SUBSTATI 0N CIRQUIT S Filed Jung 1, 1925 INVENTORS ZWMMQEDM BY iii. 7 c

"6 ATTORNEY Patented Apr. 12,

UNITED STATES PATENT OFFICE.

FREDERICK W. MQGKOWN, OF ENGLEVJOOD, AND EGINHARD DIETZE, OF WESTFIELD, NEW JERSEY, ASSIGNORS TO AMERICAN TELEPHONE AND TELEGRAPH COMPANY,

A CORPORATION OF NEW YORK.

TELEPHONE SUBSTATION CIRCUITS.

Application filed June 1,

This invention relates to circuit arrangements for signaling systems wherein signals may be either transmitted from or received at the same station. In its more specific aspect this invention is embodied in a subscribers telephone station hereinafter termed, in accordanc c with common usage,

a substation, and more particularly in the combination of a substation and a telephone Set.

The object of this invention is to provide an economical signaling circuit arrangement which in co-operat1ve combinatlon with a similar and equal slgnallng circuit arrangement at the distant e shall deliver a large nd of a telephone line, proportion of energy to the receiving apparatus of said signaling circuit arrangement a A further obJect 1s t the distant end of the to provide a substation consisting of a transmitter and a receiver of the electromagnetic type, having ity of windings and arranged for twoway communicat a plurallnvariable ion over a hne havmg minimum number of elements.

The simplest form of substation for invariable two-way commumcation is that in which the receiver and transmitter are connected 1n series wlth each other across a telephone line, the tr ansmitter and receiver each having a resistance which is equal to one-half of the line an arrangement the impedance. In such over-all efiiciency 1s a maximum for an invariable set, even though per cent of the energy dellvered by the line to the substation mitter and only 50 per cent available at the trans is wasted in the transof the energy mitter reaches the line.

Further, such an arrangement labors under the disadvantage of full side tone.

In the types of substation circuits now 1n use, the transmitter h about one-twelfth as a resistance which is of the line impedance.

Such a transmitter cannot be employed in the simple series circuit if the conditions for maximum energy ti-a1 ister are to be satisfied.

1925. Serial No. 34,163.

To improve-the efficiency of the substation it has been customary to employ a transformer or induction coil in the substation circuit in order to step up the transmitter resistance. In the arrangements of the present invention, however, the induction coil has been eliminated and its function has been relegated to the receiver.

The invention will be more fully understood by reference to the accompanying drawing, in which Figure 1 is a substation having a minimum number of elements, Fig. 2 is a modification of Fig. 1 showing the rlnging circuit, Figs. 3 and i are further modifications of Figs. 1 and 2 designed for side tone reduction on short loops, and Figs. 5 and 6 are other modifications arranged to effect lower side tone or short loops.

In order to illustrate the scope of the invention and elucidate the principles on which the embodiments rest, a general theoretical discussion will now be given which applies to the substation shown in the drawing of this specification. In this discussion 2111C the equations and formulae included in the specification, the subscripts T, L and R will refer to transmitter, line and receiver respectively. Thus, I I and I will denote the currents flowing in the transmitter, line and receiver, respectively, while Z will denote the impedance of the transmitter, Z, the impedance of the line, etc. I

Consider the substation, illustrated in Fig. 1, consisting of a transmitter and receiver connected to a line, of given impedance. In practice, the line connects two similar and equal substations between which communication is established. For transmitting, consider the transmitter as having a voltage E in series therewith. By Kirchofi s laws,

L A+ L+ T) M T) :E

and

I (Z Z +113 E In the foregoing expressions A and B refer to the two windings of the electromagnetic receiver used in the substation, Z, and Z refer to the self-impedances of said wind ings, and Z refers to the mutual impedance of said windings. Solving these equations simultaneously, (bearing in mind that the impedanceof a receiver having two windings; A and Bflwhich are-series aiding is Z +Z +2Z the current in the line, I is b E(ZB+ZM) (Z'A' Z1; ia-( 13 'T')---( 'M- T) and the current in winding B, T is In deriving the values of the currents in the re'siiective branches when signals are be mg received at the substat ionyconsider a voltage 6 to be in series with-the line Lythen IIL(ZM -ZT)I+IB(ZB+ZT) :0 The expression for the value ofthe cur- "and rent ifr'the line L, I 'when-Tsignals-"are being 'received at the substation is includes the Winding B of thereceiver 0f the substation 1s 'r'-. M) L'+ r) 3 r) M 19 Thetran sm itting efficiency of the substation' may be expressed astlre square root of ""theratio bf the power actually sent into the "line by thetra'nsmitter of thes-ub'station to "the'maximum powerthat could be"sentinto "theline by the transmitter ofthe ideal invariable substation. The transmitting" ef-' "ficiency"may bedetermined' from the expression I E, r i in which P is the power 'actuallydelivered to' the line, or

and P thepower in the ideal invariable substation circuit is determined from the expression Substituting the value of the current in-the line, I derived hereinbefore, and bearing in mind that ZM 1/ m1; approximately; the transmitting efficiency ecomes v may bee-repressed as the square root of the ratio of the power actually sent into the ref ceiver of the-subst=ation from the line to the maximum power that could be sent into the receiver of the: ideal invariable substation from the line; "The receivingeflicienoyof *the substation may be written" as inWhich P i.s't11ee1'1ergy aetually delivered to the receiver or and ton'e efficiency is in which P is the 'povver actually delivered to the receiver at the same substation-, and P is the value'obtained-'from thezexpression theimpedance of the transmitter, Z1 being merely aresi'stanee at normal telephonicfrequenciesj The value of tli'btllinli inthe receiver I having been determined herein- 7 before, the sidetone' efficiency wi'lhthen be found to be 115 I The-transmitter may be considered a generator having a fixed internal resistancewhich sends current through the receiver at the same substation and through the line in series therewith. WVhile transmitting; the energy in the receiver iswasted sofaras transmission to the substation at'the-distant end of the line is concerned. Also, while receiving, the power delivered to the transmitter is wasted. In order to have the maximum energy transfer between the line and the substation the generator element should be equal to the element to which power is being delivered. Therefore, in order to obtain the miximum energy transfer between the line and the substation while transmitting, the line should have the same impedance as the transmitter, and while receiving, the receiver should have the same impedance as the line. From the equations given hereinbefore the maximum energy transfer between the line and the substation may be obtained when the impedance of the substation, Z has the magnitude.

\ The energy ratio Y is known in the art as transmitting efficiency 2 receiving efficiency Substituting the values for the transmitting and receiving efliciencies, respectively, the energy ratio Y will be found to equal in which (9 is the receiver angle, the tangent being the ratio of the reactance component to the resistance component of a single winding of the electromagnetic receiver, (9 being of substantially equal magnitude for both of the windings.

The embodiments of this invention disclose arrangements in which the transmitter, receiver and line are effectively in parallel. Side tone may become objectionable on short loops because noises at the substation, which excite the transmitter, will produce sounds in the receiver. Furthermore, the operator talks in his own ear, and if he talks very loudly the effect of side tone may make it very uncomfortable for him. It is therefore important that on short loops the side tone be kept very low.

On inspection of the formula for the side tone efliciency it will be noted that the side tone efliciency varies in accordance with the impedance of the line, the side tone increasing as the impedance of the line increases. In arrangements of the parallel type, the

side tone may be reduced by making the impedance of the substation set higher than the impedance of the line.

Fig. 2 shows an arrangement of the parallel type, in which a condenser V is placed in series with winding B of the receiver in order to confine direct current to a path through the transmitter. Condenser V is also employed in the ringing circuit in series with ringer U. Fig. 3 shows an arrangement for short loops in which an equalizing resistance H is placed in shunt with the line L. This causes a diminution in the line current, and a still greater diminution in the side tone. Also, in this arrangement a condenser V, other than the condenser C in series with winding B, is employed in series with ringer U for ringing purposes. Fig. 4 shows a modification of Fig. 2 for short loops in which the equalizing resistance H is placed in .shunt with the line L, thus limiting the transmitted and received currents to a predetermined maximum value. Figs. 5 and 6 show modifications of the invention which are arranged to reduce side tone on short loops. In Fig. 5, one of the windings, A, is disconnected so that the transmitter, the line, and the equalizing resistance H are in series with winding B of the receiver at the substation. The presence of the equalizing resistance H in series with the line L effects a lowering of the line current, and therefore a reductlon in the side tone. In Fig. 6 no equalizing resistance is employed.

The accompanying formulae give satisfactory results and the methods by which they are derived enable one skilled in the art to compute the substation constants when clesired.

It will be understood that while certain embodiments have been specifically illustrated and described as embodied in a telephone substation, it iscapable of many and varied embodiments which render it applicable to other kinds of signaling systems. Consequently, the invention is not to be limited to the particular form and use herein disclosed but is to be limited only by the appended claims. It will be further understood that in this specification the word substation is employed in its generic sense, and that consequently, its significance is not limited to a subsc-ribers telephone station but embraces broadly a telephone station including a repeater station for relaying telephonic signals.

What is claimed is:

1. A substation circuit comprising a transmitter and a receiver of the electromagnetic type having two windings each of substantially low resistance, said instruments being connected effectively in parallel with each other with respect to signaling currents from a telephone line, the windings of said receiver being closely coupled and being so arranged with respect to said transmitter as to step-up the resistance of said transmitter to a predetermined value.

2. A substation circuit comprising a transmitter, and .an electromagnetic receiver having two windings each of substantially low resistance, said instruments being associated with a line over which signals may be transmitted and received, said transmitter being connected to the midpoint of the two windings of said receiver, the windings of said r saidresistance being placed in shunt across said line at thesuhstation in. order to diminishtheislde tone, the windings oi. said receiver -1ibeing 'related tothe transmitter so as to step i; up the transmitterresistance to a predeter-' mined value.

a 4.. In a'substation' circuit, in combination, :meansfor -transmitting signals, means i for receivingrsi-gnals, and a line to Which said =7 transmitting and receivingwmeans are connecte'd isaid receiving meanstcon'iprising tWo --.zwi1ndings having a high mutual impedance :uuandaarranged so as to attract a diaphragm in accordance withasignals that are received 1: at said-substation, said windings being of i:substantiallylowresistance and being so l0'- cated V With-:TGSPBCU towsaid transmitting .i. means as to step-up-the resistance of said transmitting means to a predetermined ;;.value. 5.1 Inna substation circuit, in combination, uctransmit-tingmeans, receiving means, ai -line,

said receiving means consisting of two Wind- .mgs each of'substantlallylow-resistance hav- 1 ,ing a high mutual impedance and arranged so as .to step-up the resistance ofs'aidtransnntting means to a predetermmed'value,

means consisting of a resistance to balance said line, ringingmeans, and means to-confine direct :current to a'path: through-said transmitting means.

6. A-fitlbiEStfltlOll circuit comprising"two branches, a receiver having plurality of windings,-. one 0t said windings being included in each of said branches, one of'said branches being adapted to beconnected to a telephone line, theother of said" branches forming a separate circuit having a"condenser serially connected thereto, and a transmitter differentially arranged With said branches, the condenser being employed to confine direct current to a path through said transmitter, thewvindings of said receiver being 0 t substantially ailowsresistance, being closely coupledandbeing ,arranged so as to step-up the resistance of said transmitter to a predetermined value.

In testimony whereof-,- We have signedour names to this specification this 2'7thday of May 1925.

FREDERICK W. MOKOWN. EGINHARD DIETZE. 

